1. Field of the Invention
The invention relates to an electro-optical character generator for the illumination of the surface of a photoconductor, in particular of a photoconductor in a high-performance printer, having a heat collector on whose bearer surface--facing the photoconductor--there is arranged a multiplicity of light-emitting elements, arranged in a row, for the illumination of the surface of the photoconductor, which are connected in heat-conducting fashion with the heat collector, and in which there is fashioned a hollow space running in the direction of the row, which is filled with a liquid whose quotient of supplied quantity of heat and change of temperature per unit of volume is greater than or equal to 2.5 kJ/dm.sup.3 K, and having a cooling means connected to the heat collector, for giving off the quantity of heat received by the heat collector to the environment.
2. Description of the Related Art
Electro-optical character generators are used primarily in copier machines and printers. By means of illumination on the surface of a photoconductor, they produce a latent charge image corresponding to the later print image, which image is colored in with toner particles. The colored-in charge image is subsequently transferred to a recording medium using a corona means, and is fixed on the surface thereof in a fixing means. Known character generators have a heat collector that extends in the longitudinal direction of the photoconductor and serves simultaneously as a bearer, on whose one bearing surface, facing the photoconductor, there is arranged a multiplicity of light-emitting elements in a row next to one another, as well as an optical means fastened to the heat collector, which sharply images the light points produced by the light-emitting elements on the surface of the photoconductor.
In addition, the character generator is equipped with a control electronics that drives the individual light-emitting elements independently of one another, by means of a multiplicity of integrated circuits (ICs), in such a way that the quantity of light respectively emitted by the elements is adjustable, and different charge states, and thus different grey gradations or, respectively, color gradations in the later print image, can be realized on the surface of the photoconductor.
As light-emitting elements, light-emitting diodes (called LEDs in the following) are suitable, particularly at an image point density of 600 dpi (dots per inch) and higher, which are fastened in groups of e.g. 128 LEDs on a common chip, known as LED arrays, in a line next to one another. Dependent on the width of the photoconductor, several such LED arrays are fastened next to one another on the bearer surface of the heat collector, the surface running in the longitudinal direction of the photoconductor, and are driven via the control electronics, which if warranted is also connected fixedly with the bearer.
In this LED array, power losses of up to 6 W per LED array can occur, so that in a high-performance printer that has for example a print width of 30 inches and uses approximately 140 such LED arrays for the illumination of the surface of the photoconductor, power losses of approximately 850 W occur. The heat quantity that arises in this way has to be removed, because the surface temperature of each LED may not exceed 50.degree. C. during operation. This is because if the surface temperature of the LED is higher, the quantity of light produced by the LEDs decreases, so that the surface of the photoconductor can no longer be illuminated by the LEDs with the same high quality.
For this reason, the light-emitting elements are connected in heat-conducting fashion with the heat collector, which collects the quantity of heat produced by the elements in order to keep the surface temperature of the elements below a critical temperature value, beyond which, as explained above, a high-quality illumination of the photoconductor is no longer possible. By means of a cooling means connected with the heat collector, the quantity of heat stored by the heat collector is given off to the surrounding environment.
It is known to adapt the actual heat capacity of the heat collector, which results from the weight-specific heat capacity of the material multiplied by the mass of the material used, to the quantity of heat produced by the light-emitting elements as lost power in such a way that this quantity of heat can be removed quickly, in order to prevent heat blockages and the resulting overheatings of the light-emitting elements. For this purpose, heat collectors are used that consist of a metal material with a high weight-specific heat capacity, such as aluminum, copper or the like. The level of the actual heat capacity is determined by the mass of the heat collector used.
Drawn or extruded full profiles made of the correspondingly suitable metal materials are used as heat collectors, which have the required mass, and thereby heat capacity, to be able to store the quantities of heat that occur. However, these heat collectors have the disadvantage that, despite the high section modulus of the full profile, due to their high intrinsic weight they bend so strongly that a uniform sharp imaging of the image points produced by the light-emitting elements on the surface of the photoconductor by the optical means is no longer possible. This problem occurs in particular in high-performance printers and copiers with broad photoconductors. The bending of the bearer of a high-performance printer that can print two paper webs with DIN A4 format or letter size format at the same time can amount to approximately 40 or 50 .mu.m. During the imaging of the image point, whose diameter is approximately 60 .mu.m, the optical means thereby produces an imaging error of 3 to 5 .mu.m, so that a sharp setting of the image points over the entire width of the surface of the photoconductor becomes impossible.
In order to increase further the cooling power of the heat collector, it is further known to fashion cooling channels in the heat collector, through which there flows a liquid that removes the heat.
Thus, EP-0 629 508 A2 specifies a character generator with light-emitting elements, on whose lower side a heat collector is fastened. A U-shaped channel is fashioned in the heat collector, which is connected with an external cooling means and forms a cooling circuit therewith, through which water flows as a cooling liquid.
In this known character generator, there is the problem that the heat collector has to be made of a material that is able to store the occurrent quantities of heat until they are transported away by the cooling liquid. The actual heat capacity of the heat collector results from the weight-specific heat capacity of the material multiplied by the mass of the material used. So that the heat collector possesses a sufficiently high heat capacity, it must be fashioned correspondingly heavily. However, this has the disadvantage that, due to its high intrinsic weight, the heat collector bends so strongly that a uniform sharp imaging of the image points produced by the light-emitting elements on the surface of the photoconductor is no longer possible. In addition, there is the problem that the heat collector is non-uniformly cooled by the water flowing through the channel as a cooling liquid. The light-emitting elements arranged near the entry opening of the channel are cooled more strongly by the cold water flowing in than are the light-emitting elements provided at the other end of the heat collector. There thus results a non-uniform temperature distribution over the length of the heat collector, which reduces the print quality of the character generator.
From JP-A 63 168 372, a character generator is known on whose upper side is fastened a transparent covering that protects the light-emitting elements. On the lower side of the character generator there is fastened a heat collector in which a channel system is fashioned. The hollow space formed by the covering with the upper side of the character generator is connected with the channel system in the heat collector. For the cooling of the light-emitting elements, a cooling liquid consisting of water and alcohol flows through the hollow space and the channel system.